The invention relates generally to methods for separating CO2 from a fluid stream.
Combustion of fossil fuels for power generation accounts for about 40% of global anthropogenic CO2 emissions. Concerns over the impact of these emissions have prompted interest in technologies for the economical capture of CO2. Membranes are an attractive option for separation because they offer potential advantages in operating energy requirements and footprint, especially if they can operate at the higher temperatures encountered in power plants. Current efforts to develop CO2-selective membranes focus on a number of approaches, including polymeric membranes which utilize a facilitated transport mechanism to transport CO2 and porous inorganic membranes which use a surface transport mechanism. Porous ceramic membranes have intrinsic thermal stability and potential for high permeance. The challenge for this class of membranes has been increasing the CO2 surface transport to a sufficiently high rate to achieve high selectivity under the operating conditions of interest.
Gas transport through porous membranes occurs through a number of mechanisms, including molecular sieving, Knudsen diffusion, and surface diffusion. Molecular sieving occurs when the pore size approaches the kinetic diameter of gas molecules, and is smaller than the kinetic diameter of the larger molecule. This results in much higher selectivities for the smaller molecule since one of the gas species is completely sieved out. In this regime, the pore diameter is typically sub-nanometer and transport tends to be an activated process, with the rate increasing exponentially with temperature. This effect is well documented in microporous silica membranes, which have been extensively studied for H2 separations. For membranes with pores that are larger than the molecular size but smaller than the mean free path of the gas molecules, transport through the pores occurs by Knudsen diffusion. According to the kinetic theory of gases, the permeance scales inversely with both temperature and the square root of molecular weight. Consequently, selectivity is proportional to the square root of the molecular weight ratio of the gases and independent of temperature.
Surface transport of CO2 through membranes has been demonstrated at room temperature in activated carbon, zeolites, and more recently in silica. (See, for example, M. B. Rao, S. Sircar. J. Membrane Sci. 85, 253 (1993); M. Hong, S. Li, J. L. Falconer, R. D. Noble. J. Memb. Sci., 307, 277 (2008); W. J. W. Bakker, F. Kapteijn, J. Poppe, J. A. Moulijn. J. Memb. Sci., 117 57 (1996); J.-H. Moon, H. Ahn, S.-H. Hyun, C.-H. Lee. Korean J. Chem. Eng., 21, 477 (2004) and C.-Y. Tsai, S.-Y. Tam, Y. Lu, C. J. Brinker. J. Membrane Sci., 169, 255 (2000).) The mechanism involves the surface diffusion of adsorbed CO2 along the pore walls. CO2 selectivity has been observed in cases when the surface transport of CO2 outweighed the contribution from Knudsen diffusion. The CO2-selective silica samples were prepared using a sol-gel method similar to that used to produce H2-selective membranes, with one important difference—the addition of an organic molecule to the sol to act as a template for porosity. This molecule was incorporated into the film and eventually burned out to produce porosity suitable for substantial surface transport. Reverse CO2/H2 selectivity up to 7 at 40° C. for templated silica has been observed, but the effect diminishes with increasing temperature due to desorption of CO2 from the pore walls. Recent efforts have focused on ways to improve selectivity through the incorporation of materials with greater CO2 affinity, such as amine groups and basic oxides. It is also believed that a suitable material may retain substantial adsorbed CO2 at higher temperatures leading to enhanced CO2 transport and CO2 selectivity at higher temperatures. To date, there have been some reports of membranes with slightly elevated CO2/N2 selectivities above 200° C., but these membranes suffer from either a decreasing CO2 flux upon heating or contain pinhole defects which dominate the flow (W. J. W. Bakker, F. Kapteijn, J. Poppe, J. A. Moulijn. J. Memb. Sci., 117 57 (1996); Y.-K. Cho, K. Han, K.-H. Lee. J. Memb. Sci., 104, 219 (1995); K. Kusakabe, K. Ichiki, S. Morooka. J. Memb. Sci., 95, 171 (1994)) Also, since N2 has a larger kinetic diameter than CO2, some of the observed enhancement may be due to molecular sieving of N2. We have previously shown that the CO2/H2 selectivity of silica membranes diminishes with increasing temperature due to reduction of CO2 flux due to desorption of CO2 from the pore walls.
Accordingly, there remains a need for membranes that can achieve CO2/H2 selectivity significantly higher than that achievable through Knudsen diffusion mechanisms at high temperatures.